V Cone Blender.

 

Principle:

• It's a “Tumbling Mixer”.

• It works on the mechanism of Diffusive Mixing.

Construction:

•  Also called “Twin Shell Blender”.

• It consists of two vessels which are attached at the ends at 70 to90 degree angle as shown in the figure.

• It is made up of stainless steel or complete plastic.

• The blender is mounted on a metal frame on which it can be rotated.

• Sometimes baffles are attached inside for thorough mixing.

• The openings are used as inlet ports while it is unloaded through the discharge port at the bottom.

Working:

• The materials to be mixed are introduced to the mixer through one of the ports.

• The mixer is loaded approximately two thirds of the volume to allow tumbling of the powders.

• When the mixer is started, it rotates along the axis and when the V is inverted, the material splits into two portions and undergoes tumbling.

• The tumbling movement during rotations causes mixing.

• The speed of the mixer plays an important role in the mixing,

• If rotations are slow: Mixing will not take place as materials will just glide over one another.

• If rotations are very fast it will cause generation of centrifugal forces hence powders will remain adhered to the walls and hence no mixing will take place.

• For better results the V cone blender needs to be operated at an optimum speed.

Uses:

1. For mixing a variety of powders and granules.

Advantages:

1. Minimum attrition and hence good for fragile granules.

2. Easy to clean, load and unload.

3. Minimum maintenance cost.

4. Available in different sizes.

Disadvantages:

1. More space is required.

2. No application of shear and hence not useful for materials with large particle size distribution.

3. Suitable for free flowing powders and granules only.

Commonly Asked Questions:

1. Define Mixing. With help of a diagram describe the principle, construction and working of the V Cone Blender.

2. Write a short note on V Cone Blender.

Double Cone Blender.

 

Principle:

• It's a “Tumbling Mixer”.

• It works on the mechanism of Diffusive Mixing.

Construction:

•  It consists of a vessel with conical ends as shown in the figure.

• All the contact parts are made up of stainless steel.

• The vessel is mounted on a metal frame on which it can be rotated.

Working:

• The materials to be mixed are introduced to the mixer through one of the ports.

• The mixer is loaded approximately two thirds of the volume to allow tumbling of the powders.

• When the mixer is started, it rotates along the axis.

• The tumbling movement during rotations causes mixing.

• The speed of the mixer plays an important role in the mixing,

• If rotations are slow: Mixing will not take place as materials will just glide over one another.

• If rotations are very fast it will cause generation of centrifugal forces hence powders will remain adhered to the walls and hence no mixing will take place.

• For better results the double cone blender needs to be operated at an optimum speed.

Uses:

1. For mixing a variety of powders and granules.

Advantages:

1. Minimum attrition and hence good for fragile granules.

2. Easy to clean, load and unload.

3. Minimum maintenance cost.

4. Available in different sizes.

Disadvantages:

1. More space is required.

2. No application of shear and hence not useful for materials with large particle size distribution.

3. Suitable for free flowing powders and granules only.

Commonly Asked Questions:

1. Define Mixing. With help of a diagram describe the principle, construction and working of the Double Cone Blender.

2. Write a short note on Double Cone Blender.

Mixing: Mechanisms of Mixing.

 

Definition

• Mixing is defined as an unit operation in which two or more components are treated in such a way  that each particle / molecule of the components lies as nearly as possible in contact with a particle / molecule of the other components.

• If this is achieved it produces a theoretical ‘ideal’ situation, called a perfect mix.

Mechanisms of mixing:

Mechanisms of Solid Mixing:

• Solids are mixed using three mechanisms as follows,

1. Convective Mixing.

2. Shear Mixing.

3. Diffusive Mixing.

1. Convective Mixing:

4. Also called “Macromixing”.

• Involves transfer of a group of particles from one part of a powder bed to another part.

2. Shear Mixing:

• Formation of shear forces by using an agitator arm or a blast of air causes movement of particles.

3. Diffusive Mixing: 

• Also called “Micromixing”.

• Involves distribution of particles over a newly formed surface.

• In this the material is tilted causing the upper surface to come down due to gravity while smaller particles diffuse to the upper layer.

Mechanisms of Liquid Mixing:

• The liquids are mixed by one or more than one of the following mechanisms,

1. Bulk Transport.

2. Turbulent Mixing.

3. Laminar Mixing.

4. Molecular Diffusion.

1. Bulk Transport:

• Involves movement of a large part of the system to another part.

• Can be achieved using paddles and blades.

2. Turbulent Mixing:

• Mixing takes place due to the formation of turbulent flow.

• In turbulent flow various smaller currents of different velocities are formed which further form smaller currents.

• Due to this formation of dividing currents, the particle mixing takes place very efficiently.

• Highly effective mechanism of mixing.

3. Laminar Mixing:

• Associated with highly viscous liquids.

• The shear force is created which breaks the liquid interface allowing movement of particles between the liquid bodies.

4. Molecular Diffusion:

• Mixing takes place at molecular levels.

• Diffusion of molecules takes place due to thermal motions of the particles.

• This process can be described quantitatively by “Fick’s Law of Diffusion” which depends on the concentration gradient at different points.

Mechanisms of Semisolid Mixing:

• The mechanisms involved depend on the nature of materials.

• The more liquid material will follow mechanisms of liquid mixing while more solid material will follow mechanisms of solid mixing.

Commonly Asked Questions.

1. Define Mixing. Write a note on mechanisms of Mixing.

Mixing: Introduction.

 

Definition

• Mixing is defined as an unit operation in which two or more components are treated in such a way  that each particle / molecule of the components lies as nearly as possible in contact with a particle / molecule of the other components.

• If this is achieved it produces a theoretical ‘ideal’ situation, called a perfect mix.

Objectives of mixing:

1. To make simple physical mixture

2. Physical change

3. Dispersion

4. Promotion of reaction.

5. To make dispersions.

6. To achieve uniformity in the composition of a product.

Types of Mixtures

• Mixtures may be divided into three types that differ in their behavior as,

1. Positive Mixtures.

2. Negative Mixtures.

3. Neutral Mixtures.

1. Positive mixtures:

• Positive mixtures are formed from materials such as gases or miscible liquids, which mix spontaneously and irreversibly by diffusion and tend to approach a perfect mix. 

• There is no input of energy required.

•  In general, such materials do not present any problems in mixing.

• e.g. Mixing of sodium chloride and sugar in water.

2. Negative mixtures:

• In negative mixtures, after mixing, the components will tend to separate out. If this occurs quickly, then energy must be continuously input to keep the components in dispersed state.

• Negative mixtures are more difficult to form and a higher degree of mixing efficiency is required.

• e.g. Calamine lotion, Emulsions and suspensions.

3. Neutral mixtures

• Neutral mixtures are static in their behavior, the components having no tendency to mix spontaneously, nor do they segregate when mixed.

• e.g. Pastes, ointments and mixed powders.

Applications of Mixing:

1. In preparation of Emulsions and Suspensions.

2. In preparation of granules in tablet manufacturing.

3. Blending of powders in capsule manufacturing.

4. In low dose drug formulation the potent drug is mixed well with diluents to form a dosage form.

5. Physical mixtures of certain drugs with cyclodextrin are found to have more bioavailabilities e.g. Nimesulide with B cyclodextrin.

Factors affecting process of Mixing:

1. Handling of Mixture:

• Once mixture is formed the vibrations in handling and transportation may cause change in the state of mixing.

2. Duration of Mixing:

• To achieve homogeneity in the mixture the mixture has to be mixed for sufficient duration of time.

• The time required for mixing can be decided by doing preformulation studies.

3. Type of Mixture used:

• Different substances are mixed by different mechanisms, the mixer should be selected appropriately for mixing particular substances.

• e.g. Solid powders are preferably mixed by using Double cone blenders while emulsions are made using Silversons homogeniser.

4. Particle Size:

• For uniform mixing particle sizes of all the components of mixture should have the same particle size.

5. Particle Shape:

• Irregular shaped particles do not produce homogeneous mixture while spherical shaped particles produce good quality mixtures.

6. Charge on Particles:

• The particles of certain substances possess static electric charges on them and hence influence mixing.

• The particles with opposite charge form a good mixture while similar charged particles do not form a mixture.

7. Particle Density:

• In a mixture the particles with heavier densities tend to settle at the bottom in a mixture while those with lesser densities accumulate on top of the mixture.

• To form a good mixture all particles should have identical densities.

8. Proportion of Mixing Material.

• The quantities of the materials to be mixed influence the process of mixing, to form a good mixture,

1. The materials with equal quantities are mixed first.

2. Then ' ‘Ascending Order” of materials as per their quantities are mixed.

Difference between Solid and Liquid Mixing:

Liquid Mixing	Solid Mixing
Mixing is achieved by formation of flow currents.	Mixing takes place by other means than formation of flow currents.
Power required for mixing is less.	Power required for mixing is more.
Well mixed products form a homogeneous mixture.	Mixed products often show two or three phases.
To study the degree of mixing smaller sizes is sufficient.	To study the degree of mixing larger sizes are required.
Equipment used for the process are called “Liquid Agitators”.	Equipment used for the process are called “Mixers or Blenders”

Commonly Asked Questions.

1. Define Mixing. Discuss Objectives and Factors affecting the process of Mixing.

2. Define mixing. Differentiate Liquid Mixing and Solid Mixing,